1. Field of the Invention
This invention relates to apparatus for transmitting bearing information and is particularly suitable for use as a coastal navigation beacon.
2. Description of the Prior Art
British Pat. No. 1,429,743 discloses apparatus for transmitting bearing information including two fixed omnidirectional antennas spaced a predetermined distance apart, means for transmitting from one antenna a composite signal comprising a combination of a VHF carrier frequency signal periodically modulated by an audio frequency signal and the unmodulated carrier frequency signal such that the modulated and unmodulated carrier frequency signals are in antiphase, means for transmitting from the second antenna the periodically modulated carrier frequency signal only, means for cyclically varying the phase of one of the transmitted signals relative to the other transmitted signal, and means for distinctively altering the audio frequency modulation of the carrier frequency signal at the commencement of each cyclic variation in phase of the one transmitted signal.
Such an apparatus will transmit signals which can be picked up by any VHF communications receiver. The transmission creates, in effect, a radio vector rotating slowly about a fixed point, i.e. the site where the two antennas are located. This vector is defined, to the VHF radio listener, by a point in time when the received frequency modulated signal has a minimum amplitude. The listener will hear first a distinctive signal which serves to identify the particular shore station being received and also working the start of a period in time. He then hears a sereis of equally spaced clearly distinguished audio signals. Beginning at the end of the distinctive timing signal the listener counts the number of regularly spaced audio signals until the amplitude of the audio frequency modulation reaches a null. The count so obtained can be directly translated into a bearing on the shore station, e.g. by reference to a table. Thus, with the aid of no more than a VHF radio receiver, a bearing table (which could be memorized) and a chart (which could also be memorized) a sailor can determine his bearing with respect to a fixed point on land and navigate a safe channel along that bearing.
The present invention proposes an apparatus which also rotates a radio vector but which is simpler in construction.